Liquefaction method comprising at least a coolant mixture using both ethane and ethylene

ABSTRACT

The invention concerns a method comprising: (a) a first step whereby natural gas ( 1 ) is subjected to a first refrigerating cycle to obtain cooled natural gas ( 4 ), and brought to a temperature less than 20° C. by a first coolant ( 201 ); a second step whereby the cooled natural gas ( 4 ) is subjected to a second refrigerating cycle wherein the cooled natural gas ( 4 ) is cooled and condensed by a second coolant ( 103 ) comprising methane, ethane, propane, and nitrogen. The second coolant ( 103 ) further contains ethylene, the total ethane and ethylene content being close to 50 mole %.

The present invention relates, in a general manner and according to afirst of its aspects, to the gas industry and in particular to a processfor liquefying natural gas.

More precisely, the invention relates to a process for liquefying anatural gas, under pressure, containing methane and C₂ and higherhydrocarbons, said process comprising:

-   -   (a) a first step I, in which the natural gas is subjected to a        first refrigerating cycle in order to obtain a cooled natural        gas and is taken to a temperature below −20° C. by a first        coolant, said first refrigerating cycle comprising a succession        of substeps (i) to (v) in which the first coolant is        subcooled, (ii) expanded, (iii) vaporized, (iv) compressed        and (v) at least partly condensed by cooling with a first        external refrigerating fluid;    -   (b) a second step II, in which the cooled natural gas is        subjected to a second refrigerating cycle in which the cooled        natural gas is cooled and condensed by a second coolant        comprising methane, ethane, propane and nitrogen, said second        refrigerating cycle comprising a succession of substeps (i)        to (vi) in which the second coolant is (i) subcooled, (ii)        expanded, (iii) vaporized, (iv) compressed, (v) cooled with a        second external refrigerating fluid and (vi) at least partly        condensed by cooling with said first coolant.

Such a process is known from the prior art. Thus, U.S. Pat. No.6,105,389 discloses a process according to the preamble described above.

Such a process has drawbacks, especially when the intake pressure of thenatural gas in the plant drops. This is because, in such a case, theliquefaction temperature of the natural gas may be significantlylowered. Thus, during conventional use of a plant according to theprocess of the prior art, the intake pressure of the natural gas isclose to 45 bar. For various reasons, for example for maintenance, thenatural gas intake pressure may drop to a pressure of about 30 bar. Inthis case, the profile of the natural gas condensation curve is modifiedand results in a relatively colder condensation temperature. Inpractice, relative variations in the level of condensation of thenatural gas will be observed in various parts of the cryogenic exchangeror exchangers compared with the situation in which the natural gas is ata pressure of 45 bar. As a corollary, the pressure needed to liquefy thenatural gas increases. Likewise, the pressure of the refrigeratingfluids may increase significantly on the output side of the compressor,consequently increasing the design pressure of the compressor and of theequipment located downstream.

Under these conditions, the invention aims to alleviate the drawbacksassociated with a reduction in the natural gas intake pressure in theplant by (i) limiting the increase in the necessary compression power,(ii) improving the heat exchange within the cryogenic exchangers withoutmodifying their structure or their area and (iii) keeping ansubstantially constant compression pressure on the output side of thecompressor.

For this purpose, the process of the invention, which is moreover inaccordance with the generic definition given in the above preamble, isessentially characterized in that the second coolant furthermorecontains ethylene.

According to a first variant of the liquefaction process of theinvention, the second coolant is separated into a relatively morevolatile first fraction and a relatively less volatile second fraction,said second fraction then being treated in accordance with substeps (i)and (ii) of step II, in order to obtain a cooled and expanded secondfraction, and then is treated in accordance with substep (iii) of stepII, said first fraction being cooled, subcooled, expanded, vaporized andthen mixed with the cooled and expanded second fraction.

Preferably, the natural gas liquefaction process according to theinvention uses a natural gas at a pressure of below 40 bar.

Preferably, the natural gas liquefaction process according to the firstvariant of the invention uses a natural gas at a pressure of below 45bar.

At least one of the first and second external refrigerating fluids maybe a fluid available at ambient temperature.

At least a first expansion turbine, preferably coupled to a generator,may be used for the treatment of the cooled second coolant at substep(ii) of step II.

Advantageously, the first coolant may consist of a container mainly ofethane and propane.

Preferably, the first coolant may consist mainly of a hydrocarboncontaining three carbon atoms, propane or propylene.

The invention will be better understood and other objects, features,details and advantages thereof will become more clearly apparent overthe course of the description that follows, with reference to theappended schematic drawings given solely by way of non-limiting exampleand in which:

FIG. 1 shows a schematic functional diagram of a plant according to onepossible embodiment of the invention;

FIG. 2 shows a schematic functional diagram of a plant according toanother possible embodiment of the invention.

In both these figures, it should in particular be noted that the symbols“GT” stands for “gas turbine” and “EG” stands for “electric generator”.

For the sake of clarity and concision, the lines used in the plants ofFIGS. 1 and 2 will be identified by the same reference numbers as thegaseous and/or liquid fractions that are flowing therein.

The plant shown in FIG. 1 is for liquefying a natural gas 1, underpressure, containing methane and C₂ and higher hydrocarbons. In a firststep I, the natural gas 1 is subjected to a first refrigerating cycle inorder to obtain a cooled natural gas 4 and is taken to a temperaturebelow −20° C. by a first coolant 201 typically comprising ethane,propane and butane.

The first coolant 201 is, in a succession of substeps (i) to (v):

-   -   (i) subcooling by the first coolant 201 passing through a        cryogenic exchanger E1, in order to obtain a stream 203. The        stream 203 is divided into a stream 204 and a stream 205. The        stream 205 is subcooled in a cryogenic exchanger E2, in order to        obtain a stream 206. The stream 206 is divided into a stream 207        and a stream 208.    -   The stream 208 is subcooled in a cryogenic exchanger E3, in        order to obtain a stream 209;    -   (ii) expansion: this is accomplished by each of the streams 204,        207 and 209 passing through a respective expansion valve V1 to        V3, in order to obtain respective expanded streams 219, 214 and        210;    -   (iii) vaporization: the streams 219, 214 and 210 are each        respectively vaporized in the cryogenic exchangers E1 to E3, in        order to deliver respective vapor streams 220, 215 and 211. Each        of these streams 220, 215 and 211 passes through a respective        tank V203, V202 and V201, in order to deliver the respective        streams 221, 216 and 212;    -   (iv) compression: the streams 221, 216 and 212 each feed a        compressor K201 comprising a plurality of stages denoted by        K201-1 to K201-3. The streams 212, 216 and 221 feed the        compressor K201 onto the respective stages K201-1, K201-2 and        K201-3, which have a progressively higher intake pressure. The        compressor K201 delivers a stream 223 at its high-pressure stage        K201-3; and    -   (v) the stream 223 is at least partly condensed by cooling with        a first external refrigerating fluid E201 in order to deliver a        stream 224, this first external refrigerating fluid possibly        being especially water or air. The stream 224 is completely        condensed by a third external refrigerating fluid E202 and        stored in a tank V204. The stream 200 is withdrawn from the tank        V204 and cooled with a fourth external refrigerating fluid E203,        in order to produce the first coolant 201.

In a second step II, the cooled natural gas 4 is subjected to a secondrefrigerating cycle in which the cooled natural gas 4 is cooled andcondensed by a second coolant 103 comprising methane, ethane, propane,nitrogen and ethylene.

The second refrigerating cycle comprises a succession of substeps (i) to(vi) in which the second coolant 103 is:

-   -   (i) subcooled by passing through a cryogenic exchanger E4 in        order to deliver a stream 104;    -   (ii) expanded by passing through an expansion turbine T101        coupled to an electric generator, denoted by EG. The turbine        T101 produces a stream 105, which is then expanded in an        expansion valve D4. The latter produces a stream 106;    -   (iii) the stream 106 is vaporized in the cryogenic exchanger E4,        which delivers the stream 107. The latter passes into a tank        V10, to give the stream 108;    -   (iv) the stream 108 feeds a compressor K101 at a low-pressure        stage K101-1. The latter produces a medium-pressure stream 109        that is cooled by exchange with a stream E101, to give a cooled        stream 110. The stream 110 is then introduced at a        medium-pressure stage of the compressor K101, at the intake of        the stage K101-2. The latter produces a stream 111;    -   (v) the stream 111 is cooled by exchange with a second external        refrigerating fluid E102 in order to produce the stream 100;        and, finally,    -   (vi) the stream is at least partly condensed by cooling with        said first coolant 201, during its successive passage through        the exchangers E1 to E3.

Referring to FIG. 2, the plant shown is also intended for liquefying anatural gas 1, under pressure, containing methane and C₂ and higherhydrocarbons. In a first step I, the natural gas 1 is subjected to afirst refrigerating cycle, in order to obtain a cooled natural gas 4,and is taken to a temperature below −20° C. by a first coolant 201typically comprising ethane, propane and butane. This cycle is identicalin its operation to that described in the case of FIG. 1. It istherefore unnecessary to describe it again.

The plant shown also includes a second refrigerating cycle having manysimilarities with that described in the case of FIG. 1. The differencesare mentioned below:

The second coolant 103 is separated, in a tank V102, into a relativelymore volatile first fraction 115 and a relatively less volatile secondfraction 119.

The second fraction 119 is then treated in accordance with substeps (i)and (ii) of step II, as described above, in order to obtain a cooled andexpanded second fraction 122.

Thus, this cooled and expanded second fraction 122 is obtained bycooling the second fraction 119 in a cryogenic exchanger E4 thatdelivers a fraction 120. The latter is expanded in a turbine T101, whichproduces an expanded stream 121. The latter stream 121 is expanded in avalve D4, which produces the cooled and expanded second fraction 122.

The cooled and expanded second fraction 122 is then mixed with afraction 118 to give a stream 106. This stream 106 is vaporized in theexchanger E4, to produce the stream 107 that feeds, via a tank V101, alow-pressure stage K101-1 of a compressor K101.

The first fraction 115 is cooled in the exchanger E4, which delivers astream 116. The latter is subcooled by passing through an exchanger E5that produces a stream 104. The stream 104 is expanded by passingthrough a turbine T102, which produces an expanded stream 105.

Next, the stream 105 is expanded in a valve D5, which produces a stream117. The latter is vaporized in the exchanger E5, which produces thestream 118. The stream 118 is then mixed with the cooled and expandedsecond fraction 122, in order to produce the stream 106.

Unlike FIG. 1, the compressor K101 comprises three compression stages,denoted by K101-1 to K101-3. Between each compression stage, thecompressed gas is cooled by a respective fluid E101 to E103.

According to a modeling of the operation of the plants shown in FIGS. 1and 2, the natural gas 1 feeds the plant with an input of 694936 kg/h.It is composed of 0.1% nitrogen, 93.8% methane, 4% ethane, 1% propane,0.5% isobutane, 0.5% n-butane and 0.1% isopentane. Its temperature is30° C.

The first coolant 201 is composed of 0.5% methane, 49.5% ethane, 49.5%propane and 0.5% isobutane.

The two tables below show the advantages of incorporating ethylene intothe second coolant 103.

Table 1 relates to a plant operating according to FIG. 1 and Table 2relates to a plant operating according to FIG. 2.

TABLE 1 Process with 2 coolants as a mixture, without phase separationPressure of the natural gas bar 45 40 35 30 Case without ethylene in thesecond coolant Composition of the coolant Nitrogen mol % 6.00 6.00 6.006.00 Methane mol % 43.50 44.50 47.40 52.00 Ethane mol % 49.50 48.5045.60 41.00 Ethylene mol % 0.00 0.00 0.00 0.00 Propane mol % 1.00 1.001.00 1.00 Total 100.00 100.00 100.00 100.00 Pressure: stream 108 bar2.85 2.85 2.85 2.85 Pressure: stream 100 bar 47.98 48.49 50.05 52.50Power of the compressor K101 kW 83005 87179 93995 103893 Power of thecompressor K201 kW 87952 89063 91029 94027 Total power kW 170957 176242185024 197920 Case with ethylene in the second coolant 103 Compositionof the coolant Nitrogen mol % 6.00 6.00 6.00 6.00 Methane mol % 43.5043.50 43.50 43.50 Ethane mol % 49.50 44.50 36.50 26.00 Ethylene mol %0.00 5.00 13.00 23.50 Propane mol % 1.00 1.00 1.00 1.00 Total 100.00100.00 100.00 100.00 Pressure: stream 108 bar 2.85 2.85 2.85 2.85Pressure: stream 100 bar 47.98 47.90 47.86 47.89 Power of the compressorK101 kW 83005 86929 91453 96722 Power of the compressor K201 kW 8795289564 91901 94765 Total power kW 170957 176493 183354 191487 Savingachieved using ethylene Power saving with ethylene kW 0 −251 1670 6433Relative power saving % 0.00 −0.14 0.90 3.25 Process with 2 coolants asa mixture, with phase separation Pressure of the natural gas bar 45 4035 30 Case without ethylene in the second coolant 103 Composition of thecoolant Nitrogen mol % 3.00 3.00 3.00 3.00 Methane mol % 43.00 46.2049.70 53.90 Ethane mol % 44.00 40.80 37.30 33.10 Ethylene mol % 0.000.00 0.00 0.00 Propane mol % 10.00 10.00 10.00 10.00 Total 100.00 100.00100.00 100.00 Pressure: stream 108 bar 3.25 3.25 3.25 3.25 Pressure:stream 100 bar 43.22 46.96 51.13 56.22 Power of the compressor K101 kW105557 114547 124746 137370 Power of the compressor K201 kW 61749 6168261530 61358 Total power kW 167306 176229 186276 198728 Case withethylene in the second coolant 103 Composition of the coolant Nitrogenmol % 3.00 3.30 3.30 3.60 Methane mol % 40.00 39.70 39.70 39.40 Ethanemol % 39.00 32.00 24.00 12.80 Ethylene mol % 8.00 15.00 23.00 34.20Propane mol % 10.00 10.00 10.00 10.00 Total 100.00 100.00 100.00 100.00Pressure: stream 108 bar 3.25 3.25 3.25 3.25 Pressure: stream 100 bar41.03 42.41 43.60 45.61 Power of the compressor K101 kW 102596 107863113325 120974 Power of the compressor K201 kW 62631 63188 63929 64624Total power kW 165227 171051 177254 185598 Saving achieved usingethylene Power saving with ethylene kW 2079 5178 9022 13130 Relativepower saving % 1.24 2.94 4.84 6.61

As is apparent upon examining the results, the incorporation of ethyleneinto the second coolant accompanied by a reduction in the proportion ofmethane allows the power needed to liquefy the natural gas 1 to besignificantly reduced. The saving is greater the lower the pressure ofthe natural gas 1. In addition, it may be seen that the pressure of thestream 108 is remarkably constant in the case of a plant according toFIG. 1. As regards the plant according to FIG. 2, the incorporation ofethylene makes it possible at least to limit the increase in pressure ofthe stream 100 relative to a system not using ethylene.

The invention is therefore remarkably advantageous by limiting theconsumption of energy during the production of liquefied natural gas, inparticular when the intake pressure of the natural gas to be liquefiedis below 45 bar. This objective is achieved while still keeping thecompression pressure of the second coolant output by the compressor K101substantially constant.

1. A process for liquefying a natural gas (1), under pressure,containing methane and C2 and higher hydrocarbons, said processcomprising: (a) a first step I, in which the natural gas (1) issubjected to a first refrigerating cycle in order to obtain a coolednatural gas (4) and is taken to a temperature below −20° C. by a firstcoolant (201), said first refrigerating cycle comprising a succession ofsubsteps (i) to (v) in which the first coolant (201) is (i) subcooled,(ii) expanded, (iii) vaporized, (iv) compressed and (v) at least partlycondensed by cooling with a first external refrigerating fluid (E201);(b) a second step II, in which the cooled natural gas (4) coming fromstep I is subjected to a second refrigerating cycle in which the coolednatural gas (4) is cooled and condensed by a second coolant (103)comprising methane, ethane, propane and nitrogen, said secondrefrigerating cycle comprising a succession of substeps (i) to (vi) inwhich the second coolant (103) is (i) subcooled, (ii) expanded, (iii)vaporized, (iv) compressed, (v) cooled with a second externalrefrigerating fluid (E102) and (vi) at least partly condensed by coolingwith said first coolant (201); the second coolant (103) furthermorecontaining ethylene, the total content in ethane and ethylene beingclose to 50 mol %, the second coolant (103) being subcooled withoutphase separation; characterized in that the ethylene and ethaneproportions in the second coolant are adjusted according to the intakepressure of the natural gas, and in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, and the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, to the total content in ethylene and ethane in this same secondcoolant, expressed in mol %, is greater than 25% when the natural gas isavailable at a pressure below 35 bar.
 2. The process for liquefying anatural gas (1) as claimed in claim 1, characterized in that the ratioof the ethylene content in the second coolant (103), expressed in mol %,and the total content in ethylene and ethane in this same secondcoolant, expressed in mol %, to the total content in ethylene and ethanein this same second coolant, expressed in mol %, is equal to 26% whenthe natural gas is available at a pressure of 35 bar.
 3. The process forliquefying a natural gas (1) as claimed in claim 1, characterized inthat the ratio of the ethylene content in the second coolant (103),expressed in mol %, to the total content in ethylene and ethane in thissame second coolant, expressed in mol %, is greater than 45% when thenatural gas is available at a pressure below 30 bar.
 4. The process forliquefying a natural gas (1) as claimed in claim 3, characterized inthat the ratio of the ethylene content in the second coolant (103),expressed in mol %, to the total content in ethylene and ethane in thissame second coolant, expressed in mol %, is equal to 48% when thenatural gas is available at a pressure of 30 bar.
 5. The process forliquefying a natural gas (1), under pressure, containing methane and C2and higher hydrocarbons, said process comprising: (a) a first step I, inwhich the natural gas (1) is subjected to a first refrigerating cycle inorder to obtain a cooled natural gas (4) and is taken to a temperaturebelow −20° C. by a first coolant (201), said first refrigerating cyclecomprising a succession of substeps (i) to (v) in which the firstcoolant (201) is (i) subcooled, (ii) expanded, (iii) vaporized, (iv)compressed and (v) at least partly condensed by cooling with a firstexternal refrigerating fluid (E201); (b) a second step II, in which thecooled natural gas (4) coming from step I is subjected to a secondrefrigerating cycle in which the cooled natural gas (4) is cooled andcondensed by a second coolant (103) comprising methane, ethane, propaneand nitrogen, said second refrigerating cycle comprising a succession ofsubsteps (i) to (vi) in which the second coolant (103) is (i) subcooled,(ii) expanded, (iii) vaporized, (iv) compressed, (v) cooled with asecond external refrigerating fluid (E102) and (vi) at least partlycondensed by cooling with said first coolant (201); the second coolant(103) furthermore containing ethylene, the total content in ethane andethylene being close to 50 mol %, characterized in that the secondcoolant (103) is separated into a relatively more volatile firstfraction (115) and a relatively less volatile second fraction (119),said second fraction (119) then being treated in accordance withsubsteps (i) and (ii) of step II in order to obtain a cooled andexpanded second fraction (122), then being treated in accordance withsubstep (iii) of step II, said first fraction (115) being cooled,subcooled, expanded, vaporized and then mixed with the cooled andexpanded second fraction (122), in that the ethylene and ethaneproportions in the second coolant are adjusted according to the intakepressure of the natural gas, and in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is greater than 15% when the natural gas (1) is available at apressure below 45 bar.
 6. The process for liquefying a natural gas (1)as claimed in claim 5, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is equal to 17% when the natural gas (1) is available at apressure of 45 bar.
 7. The process for liquefying a natural gas (1) asclaimed in claim 5, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is greater than 30% when the natural gas (1) is available at apressure below 40 bar.
 8. The process for liquefying a natural gas (1)as claimed in claim 7, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is equal to 32% when the natural gas (1) is available at apressure of 40 bar.
 9. The process for liquefying a natural gas (1) asclaimed in claim 7, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is greater than 45% when the natural gas (1) is available at apressure below 35 bar.
 10. The process for liquefying a natural gas (1)as claimed in claim 9, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is equal to 49% when the natural gas (1) is available at apressure of 35 bar.
 11. The process for liquefying a natural gas (1) asclaimed in claim 9, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is greater than 70% when the natural gas (1) is available at apressure below 30 bar.
 12. The process for liquefying a natural gas (1)as claimed in claim 11, characterized in that the ratio of the ethylenecontent in the second coolant (103), expressed in mol %, to the totalcontent in ethylene and ethane in this same second coolant, expressed inmol %, is equal to 73% when the natural gas (1) is available at apressure of 30 bar.
 13. The process for liquefying a natural gas (1) asclaimed in claim 1, characterized in that at least one of the first andsecond external refrigerating fluids (E201), E102) is a fluid availableat ambient temperature.
 14. The process for liquefying a natural gas (1)as claimed in claim 1, characterized in that at least a first expansionturbine (T101) is used for the treatment of the second coolant (103) atsubstep (ii) of step II.
 15. The liquefaction process as claimed inclaim 1, characterized in that the first coolant consists mainly ofethane and of propane.
 16. The liquefaction process as claimed in claim1, characterized in that the first coolant mainly consists of ahydrocarbon containing three carbon atoms, propane or propylene.
 17. Theliquefaction process as claimed in claim 1, characterized in that thecooled natural gas (4) coming from step I has the same composition asthe feed natural gas (1).